1. Field
The present invention relates to electronic circuits which down convert the frequency of a signal.
2. Related Art
Some electronic circuits operate as signal processing systems which condition, receive and transmit signals. One type of signal processing system utilizes code division multiple access (CDMA), which is a channel access method for signal processing. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides access by frequency. Wideband Code Division Multiple Access (WCDMA) is a wideband spread-spectrum channel access method that utilizes the direct-sequence spread spectrum method of asymmetric code division multiple access to achieve higher speeds and support more signals compared to TDMA systems.
Signal processing systems which implement CDMA or WCDMA methods often include a sigma-delta modulator, which provides a digital output signal in response to receiving an analog input signal. A sigma-delta modulator oversamples the analog input signal with a sampling signal having a sampling frequency fSample that is greater than the analog input signal bandwidth B. A signal is oversampled when it is sampled at a rate greater than the Nyquist rate fN. The Nyquist rate is the minimum sampling rate required to avoid aliasing, and is equal to two times the highest frequency of the analog input signal (fN=2×B). The analog input signal is oversampled so that the digital signal is a more accurate representation thereof.
Sampling frequency fSample is typically related to a reference frequency fREF of a reference clock signal SREF. In some signal processing systems, reference frequency fREF is about 1248 MegaHertz (MHz) and 1456 MHz. However, it is sometimes desirable to have sampling frequency fSample be much lower than reference frequency fREF. For example, it is often desirable to have a sampling frequency of 104 MHz (1248 MHz/12=104 MHz), 96 MHz (1248/13=96 Mhz) and 97.067 MHz (1456/15=97.067 MHz) at a 50% duty cycle. Hence, it is desirable to down convert reference frequency fREF to provide a sampling signal with a frequency that is a fractional value of reference frequency fREF (i.e. ⅙, 1/13, 1/15).
Some methods disclose providing sampling frequency fSample by dividing reference frequency fREF by two different integer values to generate two sub-frequencies, and then averaging the two sub-frequencies. Other methods disclose providing several phase-shifted reference signals, each having frequency fREF, and then selecting desired high-to-low and low-to-high transitions to provide the sampling signal. However, both of these methods generate jitter in the sampling signal, which refers to random variations in sampling frequency fSample. The jitter can introduce noise into the digital output signals being provided by the sigma-delta modulator, which reduces the accuracy thereof.
Some methods disclose increasing reference frequency fREF to reduce jitter, and then down converting the increased reference frequency fREF to the desired sampling frequency fSample. However, increasing reference frequency fREF requires an increase in the power consumed by the signal processing system.
It is also desirable to provide sampling signals that have a desired phase difference between them. The phase difference between the sampling signals define time points at which the analog input signal is sampled. The phase difference between sampling signals can randomly change in response to the sampling signals traveling a distance. The phase difference between sampling signals can randomly change because the sampling signals are randomly skewed in response to traveling the distance. The random change in the phase difference between the sampling signals in response to skew is often uncontrollable and can cause jitter, which reduces the accuracy of the digital output signal.